More than 50 human rights organizations called Saturday for an independent investigation into the death of a well-known human rights activist in the Democratic Republic of Congo.

Amnesty International, Human Rights Watch and other groups made the appeal for an independent probe into the killing of Floribert Chebeya in an open letter to DRC President Joseph Kabila.

Chebeya was found dead Wednesday in a car in the outskirts of Congo's capital, Kinshasa. His driver is missing.

The DRC government has promised a thorough investigation. But Philip Alston, a United Nations special investigator on extrajudicial executions, cast doubt on that statement. He told reporters Friday "there is no reason at all to think there will be a meaningful investigation of that killing."

Alston says the circumstances of Chebeya's death "make it look very likely that there was government involvement."

Chebeya was head of one of the most prominent human rights organizations in Congo, Voice of the Voiceless. He had been arrested by authorities multiple times over the past two decades.

Amnesty International says Chebeya had told it on several occasions that he believed he was being followed and under surveillance by security forces.

The U.S. government said Friday that it is deeply concerned about what it called Chebeya's apparent assassination. It said it stands ready to provide forensic experts to help Congolese authorities with their probe into the activist's death.